The invention generally relates to an apparatus and method which achieve a bond between two or more materials of a composite structure, with differing coefficients of thermal expansion (CTE). In particular, the apparatus and method of the present invention produce a substantially stress-free composite structure by applying a large isostatic pressure during the actual bonding process.
Forward looking infrared (FLIR) systems are generally known, particularly for their use in connection with military aircraft which are required to fly at low altitudes and acquire targets at night for the purpose of delivering armorment or ordnance. Current state-of-the-art FLIR systems generally require two separate and distinct infrared sensing systems, which alternately operate in different modes--one system has a wide field of view that develops an IR picture looking ahead of the aircraft in order to present information to a pilot for the purpose of flying the aircraft, in what is called "pilotage" mode. This system is known as the navigation FLIR. Additionally, a separate FLIR system is located on the aircraft, in the "targeting" mode. This second FLIR system has a narrow field of view that has magnification or telescopic properties, such as a zoom capability, so that targets can be located. The targeting FLIR system normally produces a high magnification visual image which enables the pilot to survey and acquire the target in time to maneuver his aircraft for attack and weapon delivery.
Sophisticated optical imaging systems on aircraft generally require some type of sealed enclosure, particularly when exposed to the elements. When one of these optical imaging systems is mounted on an aircraft, particularly high performance military aircraft, it is extremely important to keep the size of the sealed enclosure as small as possible in order to minimize the aerodynamic effects on the aircraft. The sealed enclosure inherently includes some type of infrared window through which the internal optical imaging system can view the outside world. In order to increase system efficiency, it is necessary to make this window as large as possible in order to maximize the amount of light that can be collected for imaging.
Both the state-of-the-art FLIR systems discussed above and conventional infrared search and track (IRST) applications utilize an infrared window through which the internal optical imaging system can view the outside world. These infrared windows are subject to extremely harsh environments. Two of the more harsh operational environments are sand at approximately 470 mph and rain drops at velocities up to Mach 1.5.
One approach for providing an infrared window which can withstand these operational environments includes bonding a standard zinc selenide ZnSe window substrate to a very durable polycrystalline diamond (PCD) with a layer of bonding material. The bonding material for this process can be either a more compliant organic polymer or a more rigid and possibly more durable chalcogenide glass. In processes where the bonding material "sets" (polymerizes or freezes) at an elevated temperature, say above 150.degree. C., the difference in the coefficients of thermal expansion (CTE) between the diamond, most bonding materials, and most substrate materials leads to huge stress in the composite structure when the composite structure cools to room temperature. These stresses frequently exceed the fracture limits of the diamond, the bonding material, or the substrate material.
In order to solve this problem with conventional bonding processes, the present invention bonds a diamond and a substrate, having identical widths and lengths at room temperature and pressure, by first heating the diamond and substrate above room temperature. Since the CTE for the substrate is usually larger than that of the diamond, the substrate's length and width will both be larger at this elevated temperature than the corresponding diamond values. However, the application of isostatic pressure to the diamond and the substrate, maintaining them at the same temperature, causes both to contract. A quantity called a material's bulk modulus is the ratio of the applied pressure to the fractional decrease in volume. Since the bulk modulus for the diamond is usually greater than that of the substrate, one can find an applied isostatic pressure at which the diamond and substrate are once again the same length and width. This length and width will be different, of course, from the length and width at room temperature and atmospheric pressure.
This process is repeated at every temperature, and a critical line can thus be defined in the pressure-temperature (P-T) plane. If the pressure and temperature values are maintained on this line when the solidification of the bonding layer takes place, then the diamond and the substrate will remain substantially stress-free as long as one stays on the critical line.